Powdered tantalum, niobium, production process thereof, and porous sintered body and solid electrolytic capacitor using the powdered tantalum or niobium

ABSTRACT

A tantalum or niobium powder that can provide a porous sintered tantalum or niobium body having a large surface area and involving a lower risk of failure to form a solid electrolyte film, where a material serving as a tantalum or niobium ion source is dissolved in a molten salt, upon which a reducing agent is allowed to act, resulting in reduction to the metal state of the tantalum or niobium ions dissolved in the molten salt, to thereby obtain a tantalum or niobium powder comprising aggregated columnar particles. A tantalum or niobium powder contains radially aggregated particles formed by a plurality of columnar particles aggregated radially.

CROSS REFERENCE TO RELATED APPLICATION

The present application is filed under 35 U.S.C. §111(a), and claimsbenefit, pursuant to 35 U.S.C. §119(e)(1), of the filing date ofProvisional Application No. 60/169,941 filed Dec. 10, 1999 pursuant to35 U.S.C. §111(b).

FIELD OF THE INVENTION

The present invention relates to a tantalum or niobium powder that isuseful as a material for a positive electrode to be incorporated into asolid electrolytic capacitor and to a process for producing the powder,and more particularly to a tantalum powder that can provide a positiveelectrode endowed with low equivalent series resistance (abbreviated as“ESR” hereinafter) and high capacitance.

BACKGROUND OF THE INVENTION

Conventionally, a positive electrode in a solid electrolytic capacitoris formed of a porous sintered tantalum or niobium body having aporosity of, for example, 70% by volume. Tantalum or niobium powderserving as a raw material for producing the porous sintered tantalum orniobium body is an agglomeration of particles assuming the form of asponge containing a large number of quasi-spherical pores thatcommunicate with one another. The agglomerated particles have aspherical shape with a particle diameter of several tens to severalhundred μm.

Conventionally, tantalum powder is produced in the following manner, forexample.

First, a primary powder is prepared by way of a known method such asreduction of potassium heptafluorotantalate by sodium or reduction oftantalum pentachloride by hydrogen. Then, the thus-obtained primarypowder is subjected to washing with acid/water as needed, degassing, andheat treatment at a temperature at least 1000° C., followed bydeoxidation treatment to remove excess oxygen, to thereby obtain atantalum powder.

Subsequently, the thus-obtained tantalum powder is subjected to pressworking into a predetermined shape and then to sintering, to therebyobtain a positive electrode having a large number of pores derived fromthe pores contained in the agglomerated particles (i.e., tantalumpowder).

Nobium primary powder is produced by the same process after thereduction of potassium heptafluoroniobate by sodium or reduction ofniobium pentachloride by hydrogen. And a positive electrode can beobtained from press working and sintering the powder.

A solid electrolytic capacitor may be produced in the following manner,for example. On the surface of the porous sintered body (the positiveelectrode), a film of a solid electrolyte (hereinafter called a solidelectrolyte film) is formed. Onto the film, a negative electrode formedof an Ni wire or a like material is bonded by soldering. The poroussintered body and the negative electrode are then covered integrallywith a coating resin such as flame-resistant epoxy resin.

Conventionally, manganese oxide has predominantly been used as a solidelectrolyte. A solid electrolyte film formed of manganese oxide may beproduced in the following manner, for example. First, the poroussintered body is subjected to chemical forming through a customarymethod. Then, the porous sintered body is soaked in a solution ofmanganese nitrate and pyrolyzed, to thereby form a solid electrolytefilm. Since the porous sintered body contains a large number ofquasi-spherical pores communicating with one another as described above,when the porous sintered body is soaked in the solution of manganesenitrate, the solution permeates through the pores on the surface of thesintered body, reaching the pores inside the sintered body thatcommunicate with the pores on the surface, and then to the pores onanother portion of the surface of the sintered body. In this manner, thesolution of manganese nitrate permeates the entirety of the poroussintered body. Accordingly, a solid electrolyte film having a large areais formed, enabling efficient use of the entire surface of the positiveelectrode.

Recently, as electronic apparatus and circuits have been down-sized andhave high frequency, there is an increasing demand for solidelectrolytic capacitors endowed with high capacitance and low ESR. Thecapacitance of a solid electrolytic capacitor increases with the surfacearea of the positive electrode present therein. Thus, the poroussintered tantalum body preferably has high porosity, to thereby producea solid electrolytic capacitor endowed with high capacitance.

Also, when the positive electrode present in a solid electrolyticcapacitor assembled into a CPU or power circuit of a personal computerhas an increased ESR, a failure may occur in signal processing withhigh-speed operation in the electronic circuits. Thus, ESR is animportant characteristic.

The primary cause for an increase in ESR of the positive electrode is afailure to form a solid electrolyte film.

However, an increase in porosity of tantalum or niobium powder in orderto produce a porous sintered body of higher porosity may deteriorate thestrength of the tantalum or niobium powder, causing crushing of poresduring press working and resulting in decreased porosity of the poroussintered body. Thus, the porosity of tantalum or niobium powder must beadjusted to a level that endows the tantalum or niobium powder withappropriate strength.

Also, a failure to form a solid electrolyte film is predominantly causedby the heterogeneity of the pores contained in the porous sintered body.

Accordingly, when the pores inside the porous sintered body do notcommunicate with the pores on the surface of the porous sintered body;i.e., when the pores inside the sintered body are closed andindependent, the solution of manganese nitrate does not permeate theseclosed pores, resulting in a failure to form a solid electrolyte film.Also, when a pore inside the sintered body communicates with one pore onthe surface of the sintered body but not to any other pore on anotherpart of the surface of the sintered body, pot-shaped pores having abottom are formed. The solution of manganese nitrate does notsufficiently permeate such a pore, resulting in a failure to form asolid electrolyte film. FIG. 7 is a microphotograph obtained by use of ascanning electron microscope (SEM) showing a porous sintered tantalumbody produced from conventional tantalum powder and containing closedpores and pot-shaped pores.

Generation of the closed pores and pot-shaped pores depends on factorssuch as the particle size distribution of the powder, the crushingresistance of pores present in the powder during press working (ease ofcompaction), and the state and fraction of pores inside the powder.

Recently, a new technique that utilizes conductive polymers instead ofmanganese oxide has been put into practical use. Since conductivepolymers are of large molecular size, they encounter difficulty inpermeating the pores present in the porous sintered body. Thus, moreprecise control over the pores present in the porous sintered body isrequired.

SUMMARY OF THE INVENTION

The present invention has been attained in view of the foregoing, and anobject of the present invention is to obtain a tantalum or niobiumpowder that can provide a porous sintered body having a large surfacearea and a lower risk of failure in forming a solid electrolyte film.

More specifically, an object of the present invention is to provide atantalum or niobium powder having homogeneous pores and endowed withappropriate strength.

Still more specifically, an object of the present invention is toprovide a technique that can reduce the risk of producing closed poresand pot-shaped pores.

The present inventors have found that a tantalum or niobium powder inthe form of columnar particles can be obtained through a specificproduction process, thus leading to completion of the present invention.

Accordingly, in order to solve the above-mentioned drawbacks, thepresent invention provides the following.

A first embodiment is directed toward providing a tantalum or niobiumpowder comprising aggregated columnar particles.

A second embodiment is directed toward providing a tantalum powderaccording to the first embodiment, wherein the powder contains radiallyaggregated particles formed by aggregating a plurality of columnarparticles radially.

A third embodiment is directed toward providing a tantalum powderaccording to the first or second embodiment, wherein the powder isobtained by reduction of a tantalate salt without allowing the tantalatesalt to come into direct contact with a reducing agent. And theembodiment is also directed toward providing a niobium powder accordingto the first embodiment, wherein the powder is obtained by reduction ofa niobate salt without allowing the niobate salt to come into directcontact with a reducing agent.

A fourth embodiment is directed toward providing a tantalum or niobiumpowder according to the third embodiment, wherein the reducing agentcomprises magnesium or sodium.

A fifth embodiment is directed toward providing a process for producinga tantalum or niobium powder, wherein the tantalum or niobium powder isobtained by reduction of a tantalate or niobate salt without allowingthe tantalate or niobate salt to come into direct contact with areducing agent.

A sixth embodiment is directed toward providing a process for producinga tantalum or niobium powder according to the fifth embodiment, whereinthe reducing agent comprises magnesium or sodium.

A seventh embodiment is directed toward providing a porous sinteredtantalum or niobium body formed of a tantalum or niobium powder asrecited in any one of the first through fourth embodiments.

An eighth embodiment is directed toward providing a solid electrolyticcapacitor, which contains a positive electrode formed of a poroussintered tantalum or niobium body as recited in the seventh invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustrative view of an apparatus used in a productionprocess of a tantalum or niobium powder according to a first embodimentof the present invention.

FIG. 2 is an illustrative view of an apparatus used in a first step of aproduction process of a tantalum powder according to a second embodimentof the present invention.

FIG. 3 is an illustrative view of an apparatus used in a second step ofthe production process of a tantalum powder according to the secondembodiment of the present invention.

FIG. 4 is a photograph of an example of a tantalum powder according tothe present invention obtained by use of SEM.

FIG. 5 is a photograph of an example of a niobium powder according tothe present invention obtained by use of SEM.

FIG. 6 is a chart showing the results of the composition identificationby X-rays (EDX) of an example of a tantalum powder according to thepresent invention.

FIG. 7 is a microphotograph obtained by use of an SEM, showing anexample of a porous sintered tantalum body formed of a conventionaltantalum powder.

DETAILED DESCRIPTION OF THE INVENTION

The tantalum powder according to the present invention may, for example,be obtained through a production process comprising the following steps.

1. A material serving as a tantalum ion source is dissolved in a moltensalt (a medium).

2. A reducing agent is allowed to act upon the molten salt.

Accordingly, the reducing agent reduces the tantalum ions dissolved inthe molten salt to the metal state, causing the crystallization andprecipitation of a tantalum powder in the molten salt. By-productsformed during reduction are dissolved in the molten salt, and thus theactivity of the by-products is suppressed, leading to excellentreactivity. Also, the tantalum powder is surrounded by the molten salt,to thereby reduce the risk of an increase in particle size of thepowders caused by collision and aggregation. Thus, powders of arelatively small size can be obtained, advantageously resulting in alarger surface area.

3. The tantalum powder is recovered from the molten salt.

4. Ions derived from a reducing agent present in the molten salt arerecovered and subjected to electrolysis or a like process for recyclingas a reducing agent.

The niobium powder is obtained using the same method only by changingthe tantalum material to niobium material.

Conventionally, a tantalate or niobate salt and a reducing agent havebeen allowed to come into direct contact with each other, such as beingmixed. However, as described above, the method in which a tantalate orniobate is reduced without being allowed to come into direct contactwith a reducing agent refers to the following steps of: dissolving atantalate or niobate salt in a medium (a molten salt); allowing aliquefied or vaporized reducing agent to act upon the medium; andreducing the tantalate or niobate salt contained in the medium by way ofthe medium.

In the present invention, examples of suitable raw materials includepotassium heptafluorotantalate and sodium heptafluorotantalate, whichhave conventionally been used in the tantalum metal refining industry.In addition, halides of tantalum such as tantalum pentachloride,tantalum pentafluoride, and tantalum pentaiodide may also be used bysuitably selecting the amount to be used in accordance with solubilityin the molten salt.

As a raw material of niobium, potassium heptafluoroniobate, sodiumheptafluoroniobate can be used, or halides of niobium such as niobiumpentachloride; niobium pentafluoride and niobium pentaiodide may also beused by selecting the amount to be used by suitably selecting the amountto be used in accordance with solubility in the molten salt.

Examples of reducing agents include alkali metals such as sodium andpotassium, and alkaline earth metals such as calcium and magnesium,which can thermodynamically reduce tantalum or niobium ions to the metalstate. Preferred redu cing agents are those which satisfy the followingconditions:

(1) The reducing agent is soluble in the molten salt.

(2) The reducing agent and tantalum (or niobium) are not soluble in eachother.

(3) Salts such as chlorides, fluorides, and iodides of the reducingagents which are by-products of the reaction, are soluble in the moltensalt.

(4) There is a large difference in specific gravity between the reducingagent and the molten salt.

(5) After completion of reaction, only the by-products of the reducingagent may be recycled through electrolysis.

(6) The reducing agent is inexpensive and is abundantly available as aresource.

Examples of reducing agents that satisfy the above conditions includesodium and magnesium. Magnesium is preferred because it can easily bereclaimed industrially.

Molten salts preferably satisfy the following conditions:

(1) The molten salt is thermochemically stable against a reducing agentand tantalum (or niobium) powder, and serves as a medium for chemicalreaction.

(2) Since the molten salt is used at high temperature, it is endowedwith characteristics such as low vapor pressure and low volatility.

(3) The molten salt is inexpensive and is abundantly available as aresource.

(4) The molten salt has a low melting point, in view of energyconservation.

(5) In order to prevent contamination by impurities, when the materialis, for example, a chloride, the molten salt is also preferably achloride.

Examples of preferred molten salts which satisfy the above conditionsand which are stable against a reducing agent include alkali metal saltsand alkaline earth metal salts. Anions constituting the salt arepreferably selected in accordance with the condition of material asdescribed in (5). In consideration of separation and recovery of anionsand tantalum (or niobium), as well as recycling of the reducing agentafter reaction is completed, an alkali metal chloride and an alkalineearth metal chloride are preferred.

The present invention is illustrated in greater detail by reference tothe following. Unless otherwise indicated, all parts, percents, ratiosand the like are by weight.

EXAMPLES

The tantalum or niobium powder of the present invention will next bedescribed by way of a tantalum example.

First Example

FIG. 1 shows an illustrative view of an apparatus used in a productionprocess of a tantalum powder according to a first Example. Referencenumeral 1 in FIG. 1 represents a container 1 formed of a material suchas dense mullite or stainless steel. The container 1 comprises a mainbody 1 a and a lid 1 b. The main body 1 a has a hollow columnar shapewith an open top and a closed bottom. A heater 1 c is provided tosurround the main body 1 a.

A thermometer 4 and a pipe (a lance) 7 are provided through the lid 1 bfor insertion into the container 1. The pipe 7, which will be describedin detail below, is maintained closed until the start of formation of amolten salt.

Gas flow tubes 6 are also provided through the lid 1 b to substitute anatmosphere in the container 1 or evacuate the interior of the container1.

A crucible 2 is formed of dense alumina (Al₂O₃) or of magnesium oxideand has a hollow columnar shape with an open top and a closed bottom.The crucible 2 is loaded with a salt before melting and placed in themain body 1 a. The salt used in this example has a eutectic compositionof KCl—NaCl and is used in an amount of 200 g. The crucible 2 has aninternal diameter of 42 mmφ and a height of 155 mm.

Subsequently, the heater 1 c is turned on and the interior of thecontainer 1 is evacuated by way of the gas flow tubes 6. Then, the saltis subjected to vacuum drying for 1-24 hours at room temperature to 600°C. The conditions applied in this example are a temperature of 200° C.and a duration of 2 hours. By subjecting the salt to vacuum drying, themolten salt is endowed with higher stability, and the resultant tantalumpowder is prevented from being oxidized by oxygen generated from watercontained in the molten salt.

Subsequently, the interior atmosphere of the container 1 is substitutedby an inert gas such as argon gas or nitrogen gas introduced through thegas flow tubes 6, and then the temperature is elevated to from 660 to1000° C. and maintained there for from 0 to 2 hours, to thereby bringthe salt contained in the container 1 to the molten state (molten salt3). In the present example, the temperature is elevated to approximately900° C. and maintained thereat for 0.5 hours.

A crucible 9 containing a reducing agent 8 is lowered into the moltensalt 3 and maintained for 0.5 to 12 hours at from 660 to 1000° C. In thepresent example, the temperature is maintained for 0.5 hours atapproximately 900° C. The wall of the crucible 9 contains a plurality ofpores that enable a gas or liquid to flow into or out of the crucible 9.

The material for the crucible 9 is selected in view of thermodynamicstability and chemical reactivity to the reducing agent.

In the present example, since magnesium serves as a reducing agent,magnesium oxide is selected as the material for the crucible 9, becauseof its thermodynamic stability to magnesium.

In this case, however, according to the thermodynamic chemical potentialof oxygen as compared with magnesium and magnesium oxide, oxygen diffuses into the molten salt 3. Consequently, the tantalum powdercontaining oxygen is obtained. In cases where oxygen contamination isundesirable, a material that does not exhibit chemical reactivity to areducing agent is preferably selected for the crucible 9. When magnesiumis used as the reducing agent, iron is suitably selected as thematerial.

Subsequently, while the pipe 7 is inserted downward through the lid 1 binto the molten salt 3 and the temperature of the molten salt 3 ismaintained at from 900 to 950° C., a material vaporized by heating issupplied to the molten salt 3 through the pipe 7. Since the temperatureof the molten salt 3 is generally higher than the boiling point of thematerial, particles of the reducing agent in solid state at roomtemperature may be fed into the pipe 7 along with an inert gas such asargon gas or nitrogen gas, to thereby be vaporized and supplied into themolten salt 3. The material used in this example is tantalumpentachloride, which has a boiling point of approximately 240° C. Thus,the tantalum pentachloride is vaporized as soon as it is fed into themolten salt 3 maintained at approximately 900° C.

Thus, chemical reaction between the reducing agent and materialcontained in the molten salt 3 proceeds. In the present example,magnesium oxide is selected as the material for the pipe 7, for the samereason it is selected as the material for the crucible 9.

Then, the crucible 9 and the pipe 7 are lifted up out of the molten salt3, to thereby cool.

Subsequently, the molten salt 3 is washed with water flow and an acidsuch as acetic acid and then recovered, to thereby obtain a mixture ofwater and tantalum powder. The tantalum powder contained in the mixtureis separated from the water by centrifugation, filtration, or a likeprocess, and then dried, to thereby recover the tantalum powder.

The shape and size of the particles forming the tantalum powder may bespecified by an analysis such as elemental analysis by energy dispersiveX-ray spectroscopy (EDX), phase identification by means of a powderX-ray diffraction apparatus, or shape analysis by use of a scanningelectron microscope (SEM).

Second Example

A production process of a tantalum powder of a second example comprisesa first step and a second step. In the first step, a salt mixture formedof the molten salt containing tantalum ions is produced. In the secondstep, a reducing agent is allowed to act upon the salt mixture, tothereby obtain a tantalum powder. The reducing agent, molten salt, andmaterial used in second example are the same as those used in the firstexample.

FIG. 2 shows a schematic view of an apparatus used in the first step.The same elements as in FIG. 1 are denoted by the same referencenumerals, and their descriptions are omitted. In the second example, acrucible 12 is formed of dense magnesium oxide. A tip of a pipe 17 forsupplying a material is covered with a tantalum foil 17 a, through whicha vaporized material is supplied, to thereby cause disproportionation.

First, the pipe 17 is placed so as to supply a material into thecontainer 1 at a temperature of from 850 to 950° C. (900° C. in thepresent example), to thereby dissolve the material in the molten salt.Then, the pipe 17 is removed, allowing the molten salt to cool, tothereby obtain a salt mixture formed of the molten salt containingtantalum ions derived from the material.

FIG. 3 shows a schematic view of an apparatus used in the second step. Asalt mixture 22 obtained as described above (30 g in the presentexample) is placed in a crucible 23. The crucible 23 is then placed in asealed container 21 formed of stainless steel or a like material. Aheater 21 a is provided to surround the sealed container 21. Thecrucible 23 used in the present example is formed of alumina.

Also, a reducing agent 25 contained in a crucible 24 is placed in thesealed container 21. In the present example, a preferred material forthe crucible 24 is magnesium oxide or iron, in consideration ofthermodynamic stability to magnesium, the reducing agent 25. Thematerial for the crucible 24 used in the present example is magnesiumoxide.

The interior of the sealed container 21 is evacuated by use of adecompression pump or a like apparatus, and then the atmosphere in thecontainer is substituted with an inert gas such as argon gas or nitrogengas in order to prevent loss of the reducing agent due to oxidation. Thetemperature is elevated to from 850 to 950° C. and maintained thereatfor from 1 to 6 hours. In the present example, the temperature iselevated to 900° C. and maintained thereat for 3 hours.

Then, the reducing agent 25 present in the crucible 24 is vaporized. Thevaporized reducing agent fills the entire atmosphere of the sealedcontainer 21 and is allowed to act upon the salt mixture 22 present inthe crucible 23, to thereby generate tantalum powder within the saltmixture 22.

Subsequently, the salt mixture 22 is washed with water and an acid suchas acetic acid, and then recovered. Then, the same procedure asdescribed in the first Example is performed so as to obtain a tantalumpowder.

FIG. 4 shows a photograph of an example of the tantalum powder accordingto the present invention obtained by use of SEM.

A plurality of particles having the shape of a quadrangular pole areformed, and about ten to one thousand of these columnar particles arefurther aggregated radially to thereby form radially aggregatedparticles.

In the tantalum powder according to the present invention, the length ofa columnar particle is from 2 to 20 μm, and the length of a side of thequadrangle, which is a cross-section perpendicular to the longitudinaldirection of the columnar particle, is from 0.1 to 2 μm. The ratio of aside of the quadrangle to the length of a columnar particle is withinthe range of from 1/2 to 1/100, preferably from 1/5 to 1/30. When theratio is in excess of 1/2, the ratio of the columnar particle is notsignificantly different from the ratio of a conventional sphericalparticle; whereas when the ratio is lower than 1/100, the columnarparticle is likely to have deteriorated strength.

The average particle diameter of the radially aggregated particles isfrom 0.2 to 10 μm, preferably from 0.35 μm. When the diameter is smallerthan 0.2 μm, the particles are difficult to handle, whereas when thediameter is larger than 10 μm, the obtained tantalum powderdisadvantageously has a small surface area.

FIG. 5 shows a SEM photograph of an example of the niobium powderaccording to the same apparatus and method supplying niobiumpentachloride into the melting salt at 850° C., using magnesium as thereducing agent. And a plurality of particles having the shape of aquadrangular pole are formed.

In the niobium powder according to the present invention, the length ofa columnar particle is from 0.3 to 100 μm, and the length of a side ofthe quadrangle, which is a cross-section perpendicular to thelongitudinal direction of the columnar particle, is from 0.3 to 2 μm.The ratio of a side of the quadrangle to the length of a columnarparticle is within the range of from 1/2 to 1/100, preferably from 1/5to 1/30. When the ratio is in excess of 1/2, the ratio of the columnarparticle is not significantly different from the ratio of a conventionalspherical particle; whereas when the ratio is lower than 1/100, thecolumnar particle is likely to have deteriorated strength.

When a porous sintered body is produced by the method of the inventionfrom the tantalum or niobium powder formed of columnar particles, thepores present in the porous sintered body are formed by a plurality ofcolumnar particles intersecting one another. Thus, the porous sinteredbody contains pores that are not easily crushed during press working,and is endowed with a large surface area and appropriate strength.Moreover, the risk of generation of closed or pot-shaped pores is low,leading to low risk of a failure to form a solid electrolyte film.

In addition, when a tantalum powder containing radially aggregatedparticles is used, pores are always formed between adjacent particles,to thereby improve these effects.

FIG. 6 is a chart showing the results of the compositionalidentification by X-rays (EDX) of an example tantalum powder accordingto the present invention.

The X-axis of the chart indicates the energy of the X-rays in the unitof keV, with the left end representing 0.000 keV and the right endrepresenting 10.110 keV. The X-ray peaks of tantalum, which are known,are marked with the symbol Ta.

The Y-axis of the chart indicates the strength of the X-rays in the unitof count number in 100 seconds, with the lower end representing 0 andthe upper end representing 5000. The count number of the highest peak,exceeds 5000.

The chart shows that tantalum was strongly detected. Aluminumattributable to the material for a crucible or the like was detected ina small amount as an impurity.

A solid electrolytic capacitor containing the tantalum or niobium powderof the present invention may be produced in the following manner. Fromthe tantalum or niobium powder, a porous sintered body is producedthrough a conventional method as described above. On the surface of theporous sintered body (a positive electrode), a solid electrolyte filmproduced from, for example, manganese oxide is formed. Onto the film, anegative electrode formed of an Ni wire or a like material is bonded bysoldering. The porous sintered body and the negative electrode are thencovered integrally with coating resin such as fire-resistant epoxyresin.

As described above, by use of the tantalum or niobium powder of thepresent invention, a porous sintered body endowed with appropriatestrength and high porosity can be obtained. Accordingly, a solidelectrolytic capacitor of high capacitance can be obtained.

Also, the porous sintered body has a low risk of generation of closed orpot-shaped pores. Thus, the risk of failure to form a solid electrolytefilm is low, and increase in ESR of the solid electrolytic capacitor canbe prevented.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope therein.

What is claimed is:
 1. A tantalum powder comprising aggregated columnarparticles that have a quadrangular cross-section, wherein the powder isobtained by reduction of a tantalate salt without allowing the tantalatesalt to come into direct contact with a reducing agent.
 2. A tantalumpowder comprising radially aggregated particles formed by radiallyaggregating a plurality of columnar particles, wherein the length ofwhich is from 2 to 20 μm, and wherein the powder is obtained byreduction of a tantalate salt without allowing the tantalate salt tocome into direct contact with a reducing agent.
 3. A tantalum powderaccording to claim 2, wherein the reducing agent comprises magnesium orsodium.
 4. A niobium powder comprising aggregated columnar particlesthat have a quadrangular cross-section, wherein the powder is obtainedby reduction of a niobate salt without allowing the niobate salt to comeinto direct contact with a reducing agent.
 5. A niobium powder accordingto claim 4, wherein the reducing agent comprises magnesium or sodium. 6.A porous sintered tantalum body formed of the tantalum powder as claimedin claim
 1. 7. A porous sintered niobium body formed of the niobiumpowder as claimed in claim
 4. 8. A porous sintered tantalum body formedof the tantalum powder as claimed in claim
 2. 9. A porous sinteredtantalum body formed of the tantalum powder as claimed in claim
 3. 10. Aporous sintered niobium body formed of the niobium powder as claimed inclaim
 5. 11. A solid electrolytic capacitor, which comprises a positiveelectrode formed of the porous sintered tantalum body as claimed inclaim
 6. 12. A solid electrolytic capacitor, which comprises a positiveelectrode formed of the porous sintered niobium body as claimed in anyone of claims 7 and
 10. 13. A solid electrolytic capacitor, whichcomprises a positive electrode formed of the porous sintered tantalumbody as claimed in claim
 8. 14. A solid electrolytic capacitor, whichcomprises a positive electrode formed of the porous sintered niobiumbody as claimed in claim
 9. 15. A tantalum powder according to claim 1,wherein a ratio of a side of the quadrangular cross-section to thelength of the columnar particle is within the range of from 1/2 to1/100.
 16. A tantalum powder comprising radially aggregated particlesformed by radially aggregating a plurality of columnar particles,wherein the length of which is from 2 to 20 μm, and wherein the columnarparticles have a quadrangular cross-section, and a ratio of a side ofthe quadrangular cross-section to the length of the columnar particle iswithin the range of from 1/2 to 1/100.
 17. A niobium powder according toclaim 4, wherein a ratio of a side of the quadrangular cross-section tothe length of the columnar particle is within the range of from 1/2 to1/100.
 18. A tantalum powder comprising radially aggregated particlesformed by radially aggregating a plurality of columnar particles,wherein the length of which is from 2 to 20 μm, and wherein the radiallyaggregated particles are formed by radially aggregating ten to onethousand of the columnar particles.
 19. A tantalum powder comprisingradially aggregated particles formed by radially aggregating a pluralityof columnar particles, wherein the length of which is from 2 to 20 μm,and wherein the columnar particles have a quadrangular cross-section,and a length of a side of the quadrangular cross-section is from 0.1 to2 μm.
 20. A tantalum powder comprising radially aggregated particlesformed by radially aggregating a plurality of columnar particles,wherein the length of which is from 2 to 20 μm, and wherein the columnarparticles have a quadrangular ross-section, and a ratio of a side of thequadrangular cross-section to the length of the columnar particle iswithin the range of from 1/5 to 1/30.
 21. A niobium powder according toclaim 4, wherein the length of the columnar particle is from 0.3 to 100μm.
 22. A niobium powder according to claim 4, wherein a length of aside of the quadrangular cross-section is from 0.3 to 2 μm.
 23. Aniobium powder according to claim 4, wherein a ratio of a side of thequadrangular cross-section to the length of the columnar particle iswithin the range of from 1/5 to 1/30.
 24. A niobium powder comprisingradially aggregated niobium particles formed by radially aggregating aplurality of columnar particles.
 25. A niobium powder according to claim24, wherein the columnar particles have a quadrangular cross-section.26. A porous sintered niobium body formed of the niobium powder asclaimed in claim 24.